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Showing posts with label Kilns and Furnaces. Show all posts
Showing posts with label Kilns and Furnaces. Show all posts

Refractory Saggers (Saggars) and Muffles

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Saggers or Saggars are boxes or cases made of refractory materials in which pottery ware, glazed or unglazed articles are heated to higher temperature. In other words saggar is a refractory container for protecting ware during heating. Because they are kept in the sagger they do not come in contact with the flames and objectionable gases. In pottery works they form an important item and are used in large numbers. Saggars are usually made of fireclay and grog but for special use they can be made of other refractory materials.

Muffle is an enclosure in a furnace protecting the ware from the flame and products of combustion. These are also made of the same type of refractory raw materials as saggers. Muffles are generally used in small furnaces for firing small quantities of wares.

The most important property of these saggers and muffles is that they should have sufficient durability. They should also have good load bearing capacity, high mechanical strength (CCS and MOR) and also high thermal conductivity. Usually a highly refractory clay with good binding power is used so that it can take a large amount of grog. Very highly plastic clay is never suitable due to its high shrinkage. Usually two clays are used one being a refractory fireclay as base material and the other a plastic clay as a bond material. Grog is used in various sizes so that the saggers (saggars) and muffles are porous enough to withstand the thermal shocks. The grog must be made from a highly refractory fireclay.

Since the proportion of grog is larger than the clay so there should be a through mixing of the two. The shaping (green manufacturing) can be done by either of the following processes:
1. Hand moulding or Pneumatic Ramming
2. Casting
3. Power pressing.

The articles are kept in the saggers and these saggers are piled one on the top of the other in the furnace. One pile may have 15 saggers depending on the size. These piles are commonly called “Bungs”. The space between the two is closed by a wad of clay so that flames etc. do not enter the saggers (saggars) and the articles inside are heated by the clean heat alone.

To fire enameled ware or for firing pottery decoration etc. muffles are used. These are open at one end and are kept over a small fire box in such a way that the flames from the fuel envelope the muffle from all sides. One end remains open and this acts as a door to place things or to take them out. This is closed by a plug or door before firing.

The bottom of the saggar as well as muffle is usually made thicker than the sides and in some cases the bottom is made from a mixture containing higher percentage of refractory grog. After green manufacturing a saggar or a muffle is dried slowly and than fired to a temperature of around 1350OC.

Saggers and Muffles should have a high mechanical strength in unburned stage and also at high temperature. They should be refractory enough to withstand the high temperature and should be able to resist the thermal shocks and should have a good thermal conductivity. Their porosity should be 20 - 25% and should have a stable volume. It is important that the walls of the saggars and muffles have a uniform thickness as otherwise strains are set due to unequal distribution of heat. These saggers (saggars) or muffles are also made of sillimanite, kyanite, silicon carbide etc. using these in the form of grog with a binding agent.

The Function of a Furnace and Kiln used in Ceramic and other Industries

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Difference between Kiln and Furnace

There are so many definitions for Kiln and Furnace. Actually kilns are an integral part of the manufacture of all ceramics, which require heat treatment, often at high temperature. The distinction of a Kiln and a Furnace is often done on the basis of user industry than on the design of the device. Generally the term kiln is used when referring to high temperature treatment of non-metallic materials such as in the ceramic, cement (cement rotary kiln), lime (lime kiln) industries etc. When melting is involved the term furnace is used as in steel manufacture (Blast Furnace, Basic Oxygen Furnace, Ladle Furnace), glass industries (Glass Melting Tank Furnace) etc.

Definition and Function
As a practical working definition, it has been proposed to restrict the term Furnace (or Kiln) to an industrial appliance, constructed to heat a material through a cycle involving temperatures exceeding 400OC. This temperature has been chosen in order to exclude a large number of industrial process in which steam is used as a medium of transferring heat. It is essential that the heat released in the space of the furnace should be so utilized that the maximum heat economy is effected. A good working furnace must therefore -
  • have very good control of temperature.
  • require a minimum amount of fuel or other energy sources and other auxiliary materials.
  • require minimum capital and maintenance costs.
It is felt that a greater use of thermodynamics in furnace design and refractory lining can lead to more accurate and closer way of thinking about such problems as those of preheat, utilization of waste heat, recirculation of the flue gases and different qualities of energy sources, etc.

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Selection of Furnace and Kiln

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Operational conditions and requirements such as optimum firing temperature and furnace atmosphere like oxidising / reducing / chemical corrosive etc. vary from product to product. Besides, there are other commercial aspects which would affect the ultimate costing of the product. 

Therefore, before selection or finalising a furnace or kiln for your venture please consider following points very carefully:

  • Robustness of furnace or kiln structure.
  • Ease of control of kiln or furnace atmosphere, temperature and flame.
  • Fuel type.
  • Fuel efficiency as it is going to impact costing of your product.
  • Ease of maintenance. Frequent breakdown or low campaign life will harm your costing.
  • Refractory design and insulation. (very important since it affects fuel efficiency of the furnace).
  • Provision of adequate draught.
  • Arrangements for recovery of waste heat (save your fuel cost).
  • Loading (charging) and unloading arrangements (important to keep further value addition to the product as minimum as possible).  
  • Furnace volume and pay load.
In selecting the furnace or kiln, the purpose for which it is required and the probable fuel consumption / electricity charges, probable cost of firing per Ton or per piece are some other points of primary importance.

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Kilns and Furnaces used in Ceramic and Refractory Industries : A General Description and Comparison

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Loaded Car coming out of a Tunnel Kiln after firing image
Fig - Tunnel Kiln exit. Kiln Car loaded with Wares (Refractory Bricks) coming out of the Kiln 

Kiln technology is quite ancient. The development of the kiln from a simple earthen trench filled with pots and fuel i.e. pit firing, to modern methods happened in stages. With the advent of the industrial age , kilns and furnaces were designed to utilize electric power and more refined fuels, including natural gas and propane instead of just wood or coal etc. The majority of large, industrial pottery kilns now use natural gas, as it is generally clean, efficient and easy to control. Modern kilns can be fitted with computerized controls, allowing fine adjustments during the firing cycle. A user may choose to control the rate of temperature increase or ramp, hold or soak the temperature at any given point, or control the rate of cooling as per the requirement. Both electric and gas kilns are common for smaller scale production in industry.

Fuel Fired Furnaces or Combustion Type of Furnaces are used to a greater extent in industry than the electric types because of high cost of electricity, but however the latter is preferred because it offers various advantages which may sometimes counter balance the cost factors for example,

(1) Cleanliness of operation.

(2) Quicker response in attaining the desired temperature.

(3) Better control of furnace atmosphere and temperature.

(4) Better cost and fuel efficiency.

Firing in Electric Furnaces is done by either of the following two methods –

(a) Induction Type: With primary or induced current flowing through resistors or by currents induced in the charge itself.

(b) Resistor Type: With the current applied to an external resistor, from which the heat is transmitted to the charge (stock material) either by conduction through a liquid or by radiation and conduction through a gaseous medium.

In Batch Type Kilns and Furnaces the temperature is practically kept constant throughout the interior. The wares or refractory bricks or any other item to be heated is laid in a certain position and remain there until the firing is completed. It is then generally removed by the same door by which it entered.

In Continuous Type Kilns and Furnaces has pre-heating and after-cooling zones, and the charged materials i.e. wares, bricks, etc. moves while it is being heated and the temperature varies from zone to zone remaining steady at the particular zone. In some cases the charge or stock material passes from over a stationary hearth, or else the hearth itself moves. The stock material passes over skids or rolls, being carried away in an incline by the force of gravity, or it is pushed through the furnace by mechanical means.

For many purposes the Rotating Hearth or Rotating Table Furnaces are very useful. The material may also be fed through the furnace by screw action in which system the whole furnace is a revolving tube with internal screw thread.

The Car-type or Periodic Furnace has a movable hearth which, however, remains stationary during the cooling period. The car is moved to a position outside the furnace for loading and unloading. This type of furnace is generally used for firing heavy or bulky shaped bricks. A similar principle is applied in the so-called Elevator Furnaces.

The Reverberatory type Coal Fired Grate Furnaces have bridge wall over which the flame sweeps into the furnace hearth.

If the flames are developed inside the heating chamber proper, and they come in direct contact with the stock material, the furnace is said to be Direct Fired type Furnace. If the flame is produced under the hearth and then sweeps up into the heating chamber then the furnace is to be Under Fired. According to the nature of the flow pattern the furnace may also be called Side Fired or Over Fired. For certain processes contact with the flue gas may be injurious to the materials being heated, in such cases the charge is sometimes, closed in a Muffle (Muffle Furnace) which are generally heated either by flue gas or by electric power.

In Batch Type Fuel Fired furnaces the products of combustion leave at a high temperature and carry with them a large amount of heat which can not be utilized directly in the furnaces. Part of this heat is utilized by Regenerators or Recuperators in pre-heating the cold stock (charge at ambient temperature) or the combustion air. Both principles are used in Tunnel Kiln in which pre-heating and cooling zones are provided along with the burning zone or firing zone as integral parts of the kiln itself. As the name suggests Tunnel kiln is a long structure in which the wares or materials which are to be fired are pushed from out-side loaded on Tunnel Kiln cars and are transported through the kiln while the temperature is increased steadily first, by the products of combustion (flue gas, hot air) until entering the firing zone of the kiln where it is fired directly or indirectly either by fuel or by electric power. So, after due pre-heating these cars carrying stocks (wares / charges) enter the combustion zone / firing zone / soaking zone / burning zone where it gets properly fired and the hot fired materials (stocks) leaving the firing zone, gives up its heat to the cooling air and thereby gets cooled gradually in the after-cooling zone before leaving the Tunnel Kiln. The following table distinguishes some aspects between the batch-type and continuous-type furnaces. In the forthcoming articles we will discuss more on some of these and other furnaces individually which are used exclusively in large to medium scale metallurgical and non-metallurgical industries.

Comparison between Batch Type and Continuous Type Furnaces

Batch Type Furnaces

Continuous Type Furnaces

Capital Investment



Flexibility of Operation



Fuel Efficiency




Firing quality inferior in bulk production

Firing quality better in bulk production

Operating Cost



Related Previous Articles

The Function of a Furnace and Kiln Used in Ceramic and Other Industries

Criteria for Selection of Furnace (and Kiln)

Classification of Kilns and Furnaces